The invention generally relates to the field of synchronous power generators, such as those used in combination with gas turbines. Specifically, the invention relates to synchronous generators having both a power and auxiliary windings.
Synchronous power generators are commonly used by power utilities to produce electrical energy. Generators generally have a magnetic rotor that is surrounded by a stationary stator having conductive windings. Rotating magnetic field from the spinning rotor creates electric current in the armature windings in a stationary stator that surrounds the rotor. The current from these windings is output as electrical power from the generator. The stator generally has two or three armature windings, each of which have an induced current. These currents are synchronous, but out-of-phase with each other. The generator produces two- or three-phase alternating current as electrical power usable by electric power utilitliy companies.
Synchronous power generators are often driven by gas turbines. Gas turbines have a rotating drive shaft that is coupled to the drive shaft and rotor of the generator. When running, the gas turbine turns the drive shaft and rotor causes the power generator to produce electricity. In these gas turbine and generator power units, the generator is commonly adapted to alternatively function as a starting motor for the gas turbine. To start the gas turbine, the generator may be temporarily operated as a motor that is powered from an auxilary electrical power source. Once the generator/motor accelerates the rotational speed of the drive shaft sufficiently to start the gas turbine, the gas turbine is started. Once started, the gas turbine begins to output power to the driving the drive shaft and the generator, and the motor is switched back to operate as a generator.
A variable frequency power supply that drives a generator as a motor to start a gas tubine is referred to as a xe2x80x9cstatic startxe2x80x9d drive. The static start variable frequency power supply applies current to the stator windings of the generator. The magnetic fields created by the current in the stator windings cause the generator rotor to turn which, in turn, powers the drive shaft. The power supply gradually increases the frequency of the current applied to the stator to increase the rotational speed of the rotor. As the rotational speed of the rotor and drive shaft increases, the turbine is accelerated to its rated starting speed, and the turbine becomes self-sustaining and generates output power to drive the generator.
The General Electric Company has previously sold and marketed gas turbine and generator power units that have xe2x80x9cstatic startxe2x80x9d capabilities. FIG. 1 illustrates a conventional three-phase synchronous generator 10 that is coupled to a static start drive 12 which provides variable frequency power to drive the generator 10 as a motor in order to start a gas turbine. The static start drive 12 is switchably coupled to the three-phase output lines 14 of the armature of the generator. The output of the generator is normally connected to a closely balanced power transmission system 16. A disconnect breaker or other switch 18 connects the static start drive 12 to the output line 14 of the generator. When the breaker 18 is closed, the static start drive 12 applies power to drive the generator as a motor and start the gas turbine.
Power to drive the static start drive 12 is provided by an auxiliary power bus 20. The static start drive provides a variable frequency power to drive the generator as a motor during the gas turbine start-up mode. Once the gas turbine is running and self-sufficient, a disconnect breaker or switch 18 disconnects the static start drive from the output power lines 14 of the generator. The generator ceases being a motor and instead becomes a generator driven by the gas turbine. Electrical power provided by the generator can be applied to the balance of the power system which requires electrical power from the generator.
The static start drive is typically formed of non-moving (hence the term static) solid-state devices such as a load-commutated inverter (LCI) or pulse-width modulated (PWM) drive formed from solid-state rectifiers, diodes and other such devices. The LCI or PWM may be used to provide a variable frequency power supply from the constant frequency power supply (typically 50 Hertz (Hz) or 60 Hz) provided from the station auxiliary power bus 20. The excitation supply 22 is also powered by the auxiliary power bus 20 and is coupled to a field winding 24 of the generator.
The static start drive system generally uses a variable frequency power supply having sufficient capacity to be compatible with the generator armature winding terminals. For existing gas turbine generator systems, the armature voltage is typically in the range of 10 kV to 20 kV. The voltage of the static start drive is typically in the range of 2.3 kV to 7 kV. This range is sufficiently close to the normal operating range of the generator armature voltage for the static start drive to be applied directly to the main power windings of the generator. Moreover, for a properly-designed static start system, the voltage and current specifications of the static start drive matches the electrical characteristics of the generator to provide the required accelerating torque to the generator rotor. This matching of the static start drive system to the armature voltage of the generator is possible for generators having normal operating ranges of 10 kV to 20 kV.
FIG. 2 shows a conventional generator having an auxiliary power winding used for exciting the generator when the generator is operating in generator mode. In particular, a generator 10 having a three-phase main power winding output 26 that provides power to a balance of the power system 16, including the power system beyond the generator terminals, such as transformers, circuit breakers, transmission lines and other electrical loads in the power system. In addition, the generator has auxiliary power windings which are represented by the outputs lines 28 to those windings. The General Electric Company has developed synchronous generators having both main power windings and auxiliary power windings. In particular, the GENERREX(trademark) excitation system includes auxiliary power windings in synchronous generators, such as is described in U.S. Pat. Nos. 4,910,421; 4,682,068 and 4,477,767.
In addition, auxiliary windings in synchronous generator/motor machines have been proposed in Naval ship propulsion systems. In particular, the main generator windings would provide power to electric motors coupled to the propeller shaft. The auxiliary windings would provide power to the shipboard power distribution system for lights, motors and other ship functions.
High voltage generators have been developed that operate at normal transmission line voltages of 40 kV to 400 kV. These generators produce output power in the range of normal transmission line voltage, which is substantially greater than the output voltage range of prior generators and well beyond the voltage range of the power supply for a static start system. High voltage generators have armature windings that operate in 40 kV to 400 kV. It is believed to be impractical to couple a static start drive (which operates in the range of 2.3 kV to 7 kV) to supply the high voltage windings in a high voltage generator while providing reasonable starting torque. Accordingly, there is a need for static start drive system which may be coupled to high voltage generators.
In addition, the static start drive system can be applied to synchronous generators which are being operated as synchronous condensers. Synchronous condensers are operated in power systems to supply reactive power to assist in maintaining desired voltage levels. The condensers provide no real power to the system. Synchronous condensers are generally applied to provide reactive power that leads real power to offset or cancel normal lagging reactive power in a power load system.
A novel system has been developed which combines a static start drive with auxiliary power windings to provide a generator with the ability to use high voltage generators to start gas turbines, serve as a synchronous condenser, and perform other functions. In one embodiment of the invention, a static start drive is connected to the auxiliary winding of a high voltage synchronous generator. The auxiliary winding has winding characteristics, e.g., such as voltage and current capabilities, that are suitable for connection to a static start drive, or other auxiliary load or excitation system. The main windings of the generator may be designed for much higher voltages and would not be suitable for a static start drive.
Applying a static start drive to an auxiliary power winding of a high voltage generator allows the static start system to be used with a high voltage generator. Starting a gas turbine using a variable frequency power supply attached to an auxiliary winding in a high voltage generator allows the static start technique to be used with a wider variety of generators, e.g., high voltage generators than was previously available. In addition, providing two windings in the generator increases the flexibility in its uses. By separating the main windings from the auxiliary windings, the characteristic of each type of windings may be selected to best suit different applications of the same generator. In particular, the auxiliary windings may be designed specifically to work in conjunction with the static start drive, and the main windings may be designed to connect to a high voltage transmission line.
Alternatively, the auxiliary windings to the static start drive may have previously existed in the generator for other purposes. For example, the GENERREX excitation system employs auxiliary windings that are separate from the main power windings and are formed by a series of conductors placed in the stator slots with the main windings. These conductors form distinctive three-phase auxiliary windings separate from the main windings. By making use of existing auxiliary windings in a generator, a generator may be converted to include a static start drive system without making major internal modifications to the generator. For example, a static start system may be added to a high voltage generator with existing auxiliary without adding additional stator windings and without having to couple the static start drive to the main stator windings of the generator. In addition, use of auxiliary windings in a high voltage generator provides a means suitable for placing balanced currents in the winding region of the stator core. The combination of conductor placement and current forms a rotating magnetic field for driving the generator as a motor.
Balanced windings and balanced currents are used to create a magnetic field that rotates within the motor or generator at synchronous speed. Windings are said to be balanced if they are geometrically identical and equally placed around the periphery of the stator core (every 120xc2x0). Currents are said to be balanced if the current in each of the phases has the same magnitude (amperes), and the time phase of the alternating currents differ by a predetermined angle, typically 120xc2x0 for a three phase winding. Balanced currents flowing in the balanced winding produce a magnetic field in the gap between the rotor and stator that is constant in magnitude and spins in synchronism with the rotor.